How do you set the scale or size of the work area for modeling objects?
If you want to change the way blender tells you the units from for example "meter" to "millimeter" or "centimeter", go in the "Scene Properties" under "Units". Select the system you want to use, either "Metric" or "Imperial".
Further down you have the option to select the length unit, as mentioned for metric like "meter" and "centimeter".